<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <style>
    .box {
      width: 300px;
      height: 300px;
      /* background-color: rgba(255, 0, 0, 0.1); */
      /* background-color: rgba(245, 18, 18, 0.5); */
      /* background-color: rgb(245, 18, 18, 0.1); */
      border: 1px solid black;
      margin: auto;
      /* opacity: 0.3; */
    }
    .first {
      background-color: pink;
    }
    .box .inner { 
      background-color: green;  
    }
  </style>
  
  <body>
    <div class="box">
      <div class="first">第一个<div> 
      <div class="inner">这是第二个</div>
    </div>
  </body>
</html>
